As usual, today morning, I ran sudo apt-get -y upgrade in my Ubuntu Linux box..
But at the end of the upgrade I got "N: Ignoring file '20auto-upgrades.ucf-dist' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension".... I googled about it to find the reason for this kind of nasty warnings...

Finally, found the reason and solution for this issue from Ask Ubuntu...

According to AskUbuntu, The reason for this warning message is, the update contains a new version of configuration file.. but if you choose system package manager to keep old one itself... then the new config file would just exists there, along with old configuration file...

So every time we use package manager, it is printing file with invalid extension warning message... However, it will not cause any issues.. We can just ignore it... But if you want to get rid of this warning message.. Just follow the bellow instructions...

We can get rid of this message by removing redundant new configuration file which came with updates..
Just execute bellow command to remove the new redundant configuration file.. So that we can get rid of warning message..

sudo rm -f /etc/apt/apt.conf.d/20auto-upgrades.ucf-dist

Now run sudo apt upgrade... This time you wouldn't get warning message..